Technologies for reproducing or recording information at high densities using laser light are known, and primarily are put into practice in optical disks.
Optical disks can be classified broadly into read-only, write-once and rewritable types. Read-only disks are used as compact disks and laser disks, whereas write-once and rewritable disks are used for documents and data files, for example. For rewritable optical disks, there are optomagnetic and phase-change type disks. Phase-change optical disks utilize the fact that irradiating the recording layer with laser light causes a reversible phase change between amorphous and crystalline (or between one crystalline and a different crystalline structure). Recording is carried out by changing at least one of the refractive index and the attenuation coefficient of a thin film by irradiating laser light. For the reproduction of signals, the amplitude of transmitted light or reflected light changes in this portion, and as a result, the change of the amount of transmitted light or of reflected light that reaches a detection system can be detected.
Phase-change optical disks ordinarily have a configuration of a dielectric layer, a recording layer, a reflective layer and a protective layer formed on a substrate. As an example of the configuration of such a disk, a first dielectric layer, a recording layer, a second dielectric layer, a reflective layer and a protective layer can be layered in this order on a substrate. A further dielectric layer also can be formed on the reflective layer. By reversing the order of those layers, it is also possible to layer them in the order of reflective layer, dielectric layer, recording layer and dielectric layer.
The following explains the role of the dielectric layers. The role of the dielectric layers is to protect the recording layer from mechanical damage, to emphasize optical changes using interference effects due to multiple reflections, to block the influence of outside air and prevent chemical changes, and to reduce roughening of the substrate surface and thermal damage of the recording layer in the case that signals are recorded repeatedly.
In the optical disks disclosed in JP H09(1997)-834298A and JP H10(1998)-275360A, a GeON or a GeN interface layer is provided between the dielectric layers and the recording layer, improving the cycle properties.
For the material of the substrate, it is possible to use glass, quartz, polycarbonate or polymethylmethacrylate. The substrate can have a smooth surface, or it can be provided with groove-shaped protrusions/recesses for tracking guidance. When seen from the side on which laser light is incident when recording or reproducing information, the farther portions, that is the protrusions on the disk substrate, are called “lands,” whereas the nearer portions, that is the recesses on the disk substrate, are called “grooves”.
For the protective layer, it is possible to use a layer made by dissolving a resin in a solvent, which is then applied and dried, or a layer made by adhering a resin plate with an adhesive.
The recording layer, the dielectric layers and the reflective layers can be formed by vacuum vapor deposition or sputtering on the transparent substrate, for example.
To allow recording and reproducing at higher densities, it has been suggested to make the laser spot smaller by increasing the numerical aperture of the objective lens of the optical system for irradiating the laser light onto the optical disk (see, for example, JP H10(1998)-154351A). In this case, for the purpose of ensuring the tilt tolerance of the disk in the recording/reproducing properties, the protective layer on the side on which the laser light is incident is made thin. For example, a 0.1 mm thick polycarbonate sheet, which is extremely thin in comparison to the 0.6 mm of the protective layer of DVD-RAMs currently on the market, is provided as the protective layer.
In making the protective layer this thin, with a layering order of first dielectric layer, recording layer, second dielectric layer and reflective layer layered on a conventional disk substrate, films had to be formed on a 0.1 mm thin sheet, but considering mass production, the handling of 0.1 mm thin sheets is difficult and impractical. To solve this problem, a manufacturing method is conceivable in which the layering order of the layers is reversed, and the reflective layer, the second dielectric layer, the recording layer and the first dielectric layer are provided on a substrate that is not a 0.1 mm thin sheet (for example, a 1.1 mm thick polycarbonate substrate). However, it was found that in this case the surface of the reflective layer, which has for example Ag as its main component, has depressions/protrusions of about 50 nm width and about 10 nm height, and if the reflective layer is layered first, the dielectric layers and the recording layer are layered on top of the depressions/protrusions of the reflective layer, so that the surface of the recording layer also has large depressions/protrusions, and a constant film thickness could not be attained. This leads to noise during recording and reproduction, and is a cause for deterioration of signal quality.
In order to solve this problem, sputtering with argon gas was performed conventionally during the formation of the Ag reflection layer, and the addition of oxygen or nitrogen to this argon gas improved the surface properties of the Ag reflective film significantly. It is thought that the addition of oxygen gas or the like during the Ag reflective layer formation is indispensable in order to improve the surface properties of the reflective layer. However, if the amount of the added gas is increased too much, then, in the case of nitrogen addition, there is the risk that the explosive chemical compound silver azide is formed. And in the case of oxygen addition, the Ag atoms are activated by the added oxygen, leading to the new problem of a phenomenon similar to the corrosion of Ag when Ag reacts with S.